The present invention relates to electronics, and, more particularly, to a controlled-gain power amplifier device which is also known as a controlled-gain power driver. The present invention applies advantageously, but is not limited to radio-frequency circuits, particularly circuits used in cellular mobile telephones.
Radio-frequency circuits used in cellular mobile telephones do not incorporate the power amplifier. The power amplifier is generally an external circuit formed using gallium arsenide (GaAs) technology. On the transmission side, the last stage incorporated into the radio-frequency circuit is a power amplifier device. The power amplifier device has a controlled gain, and is also known as a power driver. A power driver typically delivers a maximum level of only a few dBm (0 to 6 dBm) at 1 or 2 GHz depending on the frequency band. The power level delivered by the specific power amplification circuit that is formed within the radio-frequency circuits provides about 27 to 33 dBm. The power amplification circuit is formed from gallium arsenide, for example.
Moreover, for new generation telephones using the Code Division Multiple Access (CDMA) mode which includes embedding several tens of communications within the same frequency band (e.g., 1.25 MHz), the power amplification end stage must be furnished with variable-gain control whose dynamic range may reach 20 dB. It is necessary to control the power transmitted in view of a particular average power level requirement in transmissions using the CDMA mode.
Finally, the power driver must exhibit linearity constraints, i.e., a linearity of the transfer function linking the output power to the level of the input signal. Stated otherwise, if the input signal level increases linearly, the power output must also increase linearly. The requirements for this last stage of the radio-frequency circuit depend on a compromise between current consumption, linearity, supply voltage, circuit area (influencing the cost of the silicon), and noise floor.
Such a device or controlled-gain power amplifier stage receives a signal, which usually originates from an external filter whose output impedance requires a 50 ohm matching for template compliance constraints. One therefore generally finds in series from the input to the output of a power amplifier device an input impedance matching network, a voltage/current transconductor block, gain control carried out by shunting a variable proportion of the current from the transconductor block to the output load, and a network for matching power to the input of the specific power amplification circuit (which follows the integrated radio-frequency circuit).
The design most frequently encountered uses an input transconductor block which is a common emitter transistor (single-input version) or two transistors with linked common emitters (differential-input version). These are known for their high power gain. After the input transconductor block is a pair of transistors with linked emitters, which shunt part of the current originating from the transconductor block. The transistors of the transconductor block may be conventionally biased by a decoupled current source.
In this type of structure, the linked emitters of the transistors of the current shunting means or circuit are linked to the collector of the transistors of the transconductor block, and are consequently current-driven. Moreover, the input of the device is linked to the bases of the transistors of the transconductor block. Finally, the input matching of this type of stage involves using an inductance connected between the emitters of the transistors of the transconductor block and ground (differential-input version) to form an impedance having a real part with respect to the input.
This type of controlled-gain power amplification stage exhibits numerous drawbacks and limitations. First, the greater the linearity requirement, the greater the consumption of the stage. Moreover, such a prior art structure leads to a limitation in the maximum power output. This is because the limitation in the maximum voltage swing can be applied to the output without saturating the output transistor, i.e., the transistors of the current shunting circuit.
Also, the conventional bias circuits merely intensify the above limitation by adding the breakdown voltage of the transistor forming the bias current source. Furthermore, whereas the use of an inductance connected between the emitter of the input transistor and ground (single-input version) makes it possible to form an input impedance whose real part is significant, the natural input impedance of a bipolar transistor degenerated by such an inductance depends on numerous parameters.
Hence, the input matching network reflects towards the input of the arrangement the drifting of the nominal input impedance of the transconductor block related to the process and temperature variations. This often leads to unstable optimizations which may impair the efficiency of production. It may also result in different characteristics of the various batches produced.
Moreover, the order of magnitude of the natural input impedance of a common emitter arrangement is greater than 200 ohms. A 50 ohm matching therefore causes, on crossing the input impedance matching network, an over voltage whose coefficient is in the ratio of the square root of the impedances. The matching is frequently rendered impossible if this coefficient is too large. The matching is then done by degrading its quality coefficient, i.e., by introducing losses which degrade the noise factor.
Moreover, the search for high linearity is frustrated by the fact that the rise in the degeneracy inductance results in an increase in the input impedance of the transconductor block, and hence in the voltage at the input of the device. This goes against the sought-after effect since the input transconductor is voltage-controlled. Furthermore, the use of a degeneracy inductance across the terminals of the transistors of the transconductor block is expensive in terms of silicon area.
In view of the foregoing background, it is therefore an object of the present invention to provide a controlled-gain power amplifier device offering a better linearity/current consumption compromise, as well as better control of the input impedance of the device. This is applicable to a single input structure and a differential input structure.
Another object of the present invention is to provide a saving in terms of silicon area relative to the prior art devices.
Yet another object of the present invention is to provide a stable input matching that is easy to achieve.
A further object of the present invention is also to provide an arrangement allowing separate optimization of gain, noise and linearity.
Yet a further object of the present invention is to provide a biasing that allows accurate control of the quiescent currents, and which is not sensitive to the offset voltage of the biased transistors at high current, and is not sensitive to differences of the coefficients xcex2 of these transistors. There are different values for the coefficients xcex2 of the transistors formed in the silicon, although in theory they are identical.
These and other objects, advantages, and features in accordance with the present invention are provided by a controlled-gain power amplifier device comprising voltage/current transconductor means or a voltage/current transconductor circuit, and gain control means or a gain control circuit comprising shunting means or a shunting circuit able to shunt to the output of the device, in response to a control signal, all or some of the current delivered by the transconductor circuit. A control circuit delivers the control signal.
According to a general characteristic of the invention, the device comprises at least one pair of transistors having linked emitters (bipolar transistors) or sources (field effect transistors), controlled at their base or gate by the control circuit. This pair of transistors make up both the transconductor circuit and the shunting circuit. The emitters or sources of the transistors of the pair are furthermore connected to the input of the device.
Stated otherwise, the pair of transistors with linked emitters or sources (a single-input architecture) or else the two pairs of transistors with linked emitters or sources (a differential architecture with differential input) form a single active stage ensuring both the transconductance function and the current shunting function. In contrast, these two functions in the prior art devices were carried out by different circuits.
Furthermore, these pairs of transistors are now power-driven rather than current-driven. When power driven, these transistors see an impedance equal to their own input impedance while. When current driven (prior art), the transistors of the transconductor block see an impedance which was much higher than their own input impedance. The linearity/consumption compromise is significantly improved since the device comprises only a single active stage instead of, as in the prior art, two cascaded active stages.
Moreover, the input impedance of the active stage depends essentially on the emitter resistance and possesses a negligible imaginary part, thereby rendering the input matching easy and stable. Furthermore, this input impedance is small compared with the 50 ohm input matching generally required. This leads to the obtaining of an over voltage coefficient of less than 1, thus aiding the linearity of the device. The small ratio between the input impedance of the active stage and the 50 ohm impedance allows the device to operate even more in a small signal domain, thereby promoting linearity. Lastly, the absence of degeneracy inductance allows a very substantial reduction in the surface expanse of the device.
According to one embodiment of the invention, the device may comprise a feedback resistor connected between the emitters or sources of the transistors of the pair and the input of the device. The use of such a feedback resistor, which results in the addition of a further parameter, allows separate optimization of gain, noise and linearity.
Furthermore, this feedback resistor ensures extra linearity. However, it also leads to a decrease in the gain of the device. Hence, it is preferable, for certain applications, for the feedback resistance to be between 0.5 times and 3 times the emitter resistance or source resistance of the transistors of the pair. This leads to a good linearity/gain compromise.
According to an advantageous embodiment of the invention, the device comprises a biasing circuit for biasing the transistors of the pair. The biasing circuit comprises a bias resistor connected between the emitters or sources of the transistors of the pair and ground, as well as auxiliary circuit to slave the common mode voltage across the terminals of the bias resistor to a reference value. This is done through an amplifier controlling the common mode voltage.
Such an embodiment makes it possible to bias the transistors under a minimum breakdown voltage by a feedback amplifier which controls the common mode current of the structure (low frequency loop). A peak output swing equal to 60% of the supply voltage is then possible with a supply voltage on the order of 2.7 volts and a breakdown voltage of 200 mV. This leads to other applications at higher output power, but also to applications for which the supply voltage is even lower than 2.7 volts.
It is also particularly advantageous to provide a resistor or an additional inductance connected between the emitters or sources of the transistors of the pair and the bias resistor. Such an additional load makes it possible to increase the radio-frequency impedance of the biasing circuit. This thereby makes it possible to minimize the radio-frequency signal losses in the biasing circuit.
The subject of the invention is also applicable to a cellular mobile telephone comprising a controlled-gain power amplifier device as defined above.